Silicon and semiconducting oxide thin-film transistor displays

ABSTRACT

An electronic device display may have an array of pixel circuits. Each pixel circuit may include an organic light-emitting diode and a drive transistor. Each drive transistor may be adjusted to control how much current flows through the organic light-emitting diode. Each pixel circuit may include one or more additional transistors such as switching transistors and a storage capacitor. Semiconducting oxide transistors and silicon transistors may be used in forming the transistors of the pixel circuits. The storage capacitors and the transistors may be formed using metal layers, semiconductor structures, and dielectric layers. Some of the layers may be removed along the edge of the display to facilitate bending. The dielectric layers may have a stepped profile that allows data lines in the array to be stepped down towards the surface of the substrate as the data lines extend into an inactive edge region.

BACKGROUND

This relates generally to electronic devices and, more particularly, toelectronic devices with displays that have thin-film transistors.

Electronic devices often include displays. For example, cellulartelephones and portable computers include displays for presentinginformation to users.

Displays such as organic light-emitting diode displays have an array ofpixels based on light-emitting diodes. In this type of display, eachpixel includes a light-emitting diode and thin-film transistors forcontrolling application of a signal to the light-emitting diode.

If care is not taken, the thin-film transistor circuitry of a displaymay exhibit excessive transistor leakage current, insufficienttransistor drive strength, poor area efficiency, hysteresis,non-uniformity, and other issues. It would therefore be desirable to beable to provide improved electronic device displays.

SUMMARY

An electronic device may include a display. The display may have pixelsthat form an active area. An inactive border area may extend along anedge portion of the active area. The pixels may be formed from an arrayof pixel circuits on a substrate. The substrate may be formed from arigid material or may be formed from a flexible material that bends inthe inactive area.

Each pixel circuit may include an organic light-emitting diode and adrive transistor coupled to that organic light-emitting diode. Eachdrive transistor may be adjusted to control how much current flowsthrough the organic light-emitting diode to which it is coupled and howmuch light is therefore produced by that diode. Each pixel circuit mayinclude one or more additional transistors such as switching transistorsand may include a storage capacitor.

Semiconducting oxide transistors and silicon transistors may be used informing the transistors of the pixel circuits. For example,semiconducting oxide transistors may be used as switching transistorsand silicon transistors may be used as drive transistors. There may be asingle drive transistor and one or more additional transistors per pixelcircuit.

The storage capacitors and the transistors may be formed using metallayers, semiconductor structures, and dielectric layers. The dielectriclayers may have a stepped profile that allows data lines in the array ofpixel circuits to be gradually stepped down towards the surface of thesubstrate as the data lines extend into an inactive bent edge region ofthe display. Some or all of the dielectric layers may be removed ininactive edge region to facilitate bending.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative display such as an organiclight-emitting diode display having an array of organic light-emittingdiode pixels in accordance with an embodiment.

FIG. 2 is a diagram of an illustrative organic light-emitting diodedisplay pixel of the type that may be used in an organic light-emittingdiode with semiconducting oxide thin-film transistors and siliconthin-film transistors in accordance with an embodiment.

FIG. 3 is a cross-sectional side view of illustrative thin-filmtransistor structures for a display pixel in a configuration in which asemiconducting oxide thin-film transistor has been formed using a bottomgate arrangement in accordance with an embodiment.

FIG. 4 is a cross-sectional side view of illustrative thin-filmtransistor structures for a display pixel in a configuration in which asemiconducting oxide thin-film transistor has been formed using a topgate arrangement in accordance with an embodiment.

FIG. 5 is a cross-sectional side view of illustrative thin-filmtransistor structures for a display pixel in a configuration in which asemiconducting oxide thin-film transistor has been formed using a bottomgate arrangement and in which a storage capacitor has a first electrodepatterned from the same metal layer as the gate of the semiconductingoxide thin-film transistor and a second electrode that also formstransistor source-drain electrodes in accordance with an embodiment.

FIG. 6 is a cross-sectional side view of illustrative thin-filmtransistor structures for a display pixel in a configuration in which asemiconducting oxide thin-film transistor has been formed using a bottomgate arrangement and in which a storage capacitor has been formed usinga lower electrode patterned from a layer of metal that also serves as athin-film transistor gate metal in a silicon transistor in accordancewith an embodiment.

FIG. 7 is a cross-sectional side view of illustrative thin-filmtransistor structures for a display pixel in a configuration in which asemiconducting oxide thin-film transistor has been formed using a bottomgate arrangement having three layers of interlayer dielectric interposedbetween its gate and its channel in accordance with an embodiment.

FIG. 8 is a perspective view of an illustrative display with a bent edgein accordance with an embodiment.

FIG. 9 is a cross-sectional side view of illustrative stepped dielectriclayers for a display with a bent edge in accordance with an embodiment.

FIG. 10 is a cross-sectional side view of illustrative thin-filmtransistor structures for a display in a configuration in which upperlayers of material have been removed from the display to facilitatedisplay bending in an inactive area along the edge of the display inaccordance with an embodiment.

FIG. 11 is a cross-sectional side view of illustrative thin-filmtransistor structures for a display in a configuration in which upperlayers of material have been removed from the display to facilitatedisplay bending in a bend region along the edge of the display and inwhich semiconducting oxide transistor structures do not overlap anyhydrogen-rich silicon nitride in accordance with an embodiment.

DETAILED DESCRIPTION

A display in an electronic device may be provided with driver circuitryfor displaying images on an array of pixels. An illustrative display isshown in FIG. 1. As shown in FIG. 1, display 14 may have one or morelayers such as substrate 24. Layers such as substrate 24 may be formedfrom insulating materials such as glass, plastic, ceramic, and/or otherdielectrics. Substrate 24 may be rectangular or may have other shapes.Rigid substrate material (e.g., glass) or flexible substrate material(e.g., a flexible sheet of polymer such as a layer of polyimide or othermaterials) may be used in forming substrate 24.

Display 14 may have an array of pixels 22 (sometimes referred to aspixel circuits) for displaying images for a user. The array of pixels 22may be formed from rows and columns of pixel structures on substrate 24.There may be any suitable number of rows and columns in the array ofpixels 22 (e.g., ten or more, one hundred or more, or one thousand ormore).

Display driver circuitry such as display driver integrated circuit 16may be coupled to conductive paths such as metal traces on substrate 24using solder or conductive adhesive. Display driver integrated circuit16 (sometimes referred to as a timing controller chip) may containcommunications circuitry for communicating with system control circuitryover path 25. Path 25 may be formed from traces on a flexible printedcircuit or other cable. The control circuitry may be located on a mainlogic board in an electronic device such as a cellular telephone,computer, set-top box, media player, portable electronic device,wrist-watch device, tablet computer, or other electronic equipment inwhich display 14 is being used. During operation, the control circuitrymay supply display driver integrated circuit 16 with information onimages to be displayed on display 14. To display the images on displaypixels 22, display driver integrated circuit 16 may supply correspondingimage data to data lines D while issuing clock signals and other controlsignals to supporting thin-film transistor display driver circuitry suchas gate driver circuitry 18 and demultiplexing circuitry 20.

Gate driver circuitry 18 may be formed on substrate 24 (e.g., on theleft and right edges of display 14, on only a single edge of display 14,or elsewhere in display 14). Demultiplexer circuitry 20 may be used todemultiplex data signals from display driver integrated circuit 16 ontoa plurality of corresponding data lines D. With the illustrativearrangement of FIG. 1, data lines D run vertically through display 14.Each data line D is associated with a respective column of displaypixels 22. Gate lines G run horizontally through display 14. Each gateline G is associated with a respective row of display pixels 22. Gatedriver circuitry 18 may be located on the left side of display 14, onthe right side of display 14, or on both the right and left sides ofdisplay 14, as shown in FIG. 1.

Gate driver circuitry 18 may assert gate signals (sometimes referred toas scan signals) on the gate lines G in display 14. For example, gatedriver circuitry 18 may receive clock signals and other control signalsfrom display driver integrated circuit 16 and may, in response to thereceived signals, assert a gate signal on gate lines G in sequence,starting with the gate line signal G in the first row of display pixels22. As each gate line is asserted, the corresponding display pixels inthe row in which the gate line is asserted will display the display dataappearing on the data lines D.

Display driver circuitry 16 may be implemented using one or moreintegrated circuits. Display driver circuitry such as demultiplexercircuitry 20 and gate driver circuitry 18 may be implemented using oneor more integrated circuits and/or thin-film transistor circuitry onsubstrate 24. Thin-film transistors may be used in forming circuitry indisplay pixels 22. To enhance display performance, thin-film transistorstructures in display 14 may be used that satisfy desired criteria suchas leakage current, switching speed, drive strength, uniformity, etc.The thin-film transistors in display 14 may, in general, be formed usingany suitable type of thin-film transistor technology (e.g.,silicon-based, semiconducting-oxide-based, etc.).

With one suitable arrangement, which is sometimes described herein as anexample, the channel region (active region) in some thin-filmtransistors on display 14 is formed from silicon (e.g., silicon such aspolysilicon deposited using a low temperature process, sometimesreferred to as LTPS or low-temperature polysilicon) and the channelregion in other thin-film transistors on display 14 is formed from asemiconducting oxide material (e.g., amorphous indium gallium zincoxide, sometimes referred to as IGZO). If desired, other types ofsemiconductors may be used in forming the thin-film transistors such asamorphous silicon, semiconducting oxides other than IGZO, etc. In ahybrid display configuration of this type, silicon transistors (e.g.,LTPS transistors) may be used where attributes such as switching speedand good reliability are desired (e.g., for drive transistors to drivecurrent through organic light-emitting diodes in pixels), whereas oxidetransistors (e.g., IGZO transistors) may be used where low leakagecurrent is desired (e.g., as display pixel switching transistors in adisplay implementing a variable refresh rate scheme or other scenario inwhich low leakage current is require). Other considerations may also betaken into account (e.g., considerations related to power consumption,real estate consumption, hysteresis, transistor uniformity, etc.).

Oxide transistors such as IGZO thin-film transistors are generallyn-channel devices (i.e., NMOS transistors), but PMOS devices may be usedfor oxide transistors if desired. Silicon transistors can also befabricated using p-channel or n-channel designs (i.e., LTPS devices maybe either PMOS or NMOS). Combinations of these thin-film transistorstructures can provide optimum performance for an organic light-emittingdiode display.

In an organic light-emitting diode display, each pixel contains arespective organic light-emitting diode. A schematic diagram of anillustrative organic light-emitting diode display pixel is shown in FIG.2. As shown in FIG. 2, pixel 22 may include light-emitting diode 26. Apositive power supply voltage ELVDD may be supplied to positive powersupply terminal 34 and a ground power supply voltage ELVSS may besupplied to ground power supply terminal 36. The state of drivetransistor 28 controls the amount of current flowing through diode 26and therefore the amount of emitted light 40 from display pixel 22.

To ensure that transistor 28 is held in a desired state betweensuccessive frames of data, display pixel 22 may include a storagecapacitor such as storage capacitor Cst. The voltage on storagecapacitor Cst is applied to the gate of transistor 28 at node A tocontrol transistor 28. Data can be loaded into storage capacitor Cstusing one or more switching transistors such as switching transistor 30.When switching transistor 30 is off, data line D is isolated fromstorage capacitor Cst and the gate voltage on terminal A is equal to thedata value stored in storage capacitor Cst (i.e., the data value fromthe previous frame of display data being displayed on display 14). Whengate line G (sometimes referred to as a scan line) in the row associatedwith pixel 22 is asserted, switching transistor 30 will be turned on anda new data signal on data line D will be loaded into storage capacitorCst. The new signal on capacitor Cst is applied to the gate oftransistor 28 at node A, thereby adjusting the state of transistor 28and adjusting the corresponding amount of light 40 that is emitted bylight-emitting diode 26.

The illustrative pixel circuit of FIG. 2 is just one example ofcircuitry that may be used for the array of pixels in display 14. Forexample, each pixel circuit may include any suitable number of switchingtransistors (one or more, two or more, three or more, etc.). If desired,organic light-emitting diode display pixel 22 may have additionalcomponents (e.g., one or two emission enable transistors coupled inseries with the drive transistor to help implement functions such asthreshold voltage compensation, etc.). In general, the thin-filmtransistor structures described herein may be used with the pixelcircuit of FIG. 2 or with any other suitable pixel circuits. As anexample, the thin-film transistor structures described herein may beused in six-transistor pixel circuits having three switching transistorcontrolled by two different scan lines, a drive transistor coupled inseries with an organic light-emitting diode, and two emission enabletransistors controlled by two respective emission lines and coupled inseries with the drive transistor and light-emitting diode to implementthreshold voltage compensation functions. Thin-film transistor circuitsfor pixel in display 14 may also have other numbers of switchingtransistors (e.g., one or more, two or more, three or more, four ormore, etc.) or other numbers of emission transistors (no emissiontransistors, one or more emission transistors, two or more emissiontransistors, three or more emission transistors, four or more emissiontransistors, etc.). The transistors in each pixel circuit may be formedfrom any suitable combination of silicon and silicon oxide transistorsand any suitable combination of NMOS and PMOS transistors. The pixelcircuitry of FIG. 2 is merely illustrative.

Organic light-emitting diode pixels such as pixel 22 of FIG. 2 or anyother suitable pixel circuits for display 14 may use thin-filmtransistor structures of the type shown in FIG. 3. In this type ofstructure, two different types of semiconductor are used. As shown inFIG. 3, pixel circuitry 72 may include pixel structures such aslight-emitting diode cathode terminal 42 and light-emitting diode anodeterminal 44. Organic light-emitting diode emissive material 47 may beinterposed between cathode 42 and anode 44, thereby forminglight-emitting diode 26 of FIG. 2. Dielectric layer 46 may serve todefine the layout of the pixel (e.g., alignment of the emissive material47 with respect to anode 44) and may sometimes be referred to as a pixeldefinition layer. Planarization layer 50 (e.g., a polymer layer) may beformed on top of thin-film transistor structures 52. Thin-filmtransistor structures 52 may be formed on substrate 24. Substrate 24 maybe rigid or flexible and may be formed from glass, ceramic, crystallinematerial such as sapphire, polymer (e.g., a flexible layer of polyimideor a flexible sheet of other polymer material), etc.

Thin-film transistor structures 52 may include silicon transistors suchas silicon transistor 58. Transistor 58 may be an LTPS transistor formedusing a “top gate” design and may be used to form any of the transistorsin pixel 22 (e.g., transistor 58 may serve as a drive transistor such asdrive transistor 28 in pixel 22 of FIG. 2). Transistor 58 may have apolysilicon channel 62 that is covered by gate insulator layer 64 (e.g.,a layer of silicon oxide or other inorganic layer). Gate 66 may beformed from patterned metal (e.g., molybdenum, as an example). Gate 66may be covered by a layer of interlayer dielectric (e.g., siliconnitride layer 68 and silicon oxide layer 70 or other inorganic layers ororganic material). Source-drain contacts 74 and 76 may contact opposingsides of polysilicon layer 62 to form the silicon thin-film transistor58.

Gate 66 may be formed from a metal layer GATE, source-drain terminals 74and 76 may be formed from a metal layer SD, and an additional metallayer M3 may be used to form metal via 75 to couple source-drainelectrode 74 to anode 44.

Circuitry 72 may also include capacitor structures such as capacitorstructure 100 (e.g., capacitor Cst of FIG. 2). Capacitor structure 100may have a lower electrode such as electrode 102 and an upper electrodesuch as electrode 104. Lower electrode 102 may be formed from apatterned portion of metal layer SD. Upper electrode 104 may be formedfrom a patterned portion of metal layer M3. A dielectric layer mayseparate upper electrode 104 and lower electrode 102. The dielectriclayer may be formed from a high-dielectric-constant material such ashathium oxide or aluminum oxide or may be formed from one or more otherlayers. In the example of FIG. 3, the dielectric layer separatingelectrodes 102 and 104 includes two passivation layers 106 and 108.Layers 106 and 108 may be formed from silicon oxide and silicon nitride,respectively. Other inorganic layers and/or organic layers may be usedin forming layers 106 and 108, if desired (e.g., oxide layers, nitridelayers, polymer layers, etc.).

Thin-film transistor structures 52 may include semiconducting oxidetransistors such as semiconducting oxide transistor 60. The thin-filmtransistor in structures 60 may be a “bottom gate” oxide transistor.Gate 110 of transistor 60 may be formed from a portion of metal layerGATE. The semiconducting oxide channel region of transistor 60 (channel112) may be formed from a semiconducting oxide such as IGZO. Interlayerdielectric (e.g., layers 68 and 70) may be interposed between gate 110and semiconducting oxide channel 112 and may serve as the gate insulatorlayer for transistor 60. Oxide transistor 60 may have source-drainterminals 114 and 116 formed from patterned portions of metal layer SD.

Buffer layer 122 on substrate 24 may be formed from a layer of polyimideor other dielectric. Back-side metal layer 118 may be formed undertransistor 58 to shield transistor 58 from charge in buffer layer 122.Buffer layer 120 may be formed over shield layer 118 and may be formedfrom a dielectric (e.g., an organic layer such as a polymer layer orother insulating layer).

Additional illustrative thin-film transistor circuitry 72 for pixelcircuit 22 is shown in FIG. 4. In the example of FIG. 4, oxidetransistor 60 has been formed using a “top gate” arrangement. With thisapproach, gate 110 for transistor 60 is formed from a patterned portionof metal layer M3. Metal layer M3 may also be used in forming electrode104 of capacitor 100 (as an example). Metal layer SD may be used informing electrode 102, source-drain terminals 74 and 76, andsource-drain terminals 114 and 116. Oxide transistor 60 may havesemiconducting oxide channel 112. Dielectric (e.g., passivation layers106 and 108 and/or a high-dielectric-constant material or otherinsulating material) may be interposed between channel 112 and gate 110.

In the example of FIG. 5, transistor 60 of circuitry 72 is a bottom gateoxide transistor. Dielectric layer 132 may be interposed between upperelectrode 104 and lower electrode 102 of capacitor 100. Dielectric layer132 may be formed from an inorganic insulator (e.g., silicon oxide,silicon nitride, etc.) or may be formed from a polymer layer. Layer 132may sometimes be referred to as an interlayer dielectric layer and maybe formed on top of interlayer dielectric layers 68 and 70. In capacitor100, layer 132 separates electrodes 102 and 104 from each other. Upperelectrode 104 may be formed from metal layer SD. Metal layer SD may alsobe used in forming source-drain electrodes 74 and 76 for silicontransistor 58 and source-drain electrodes 114 and 116 for oxidetransistor 60. Lower electrode 102 may be formed a metal layer that isdeposited and patterned between gate metal GATE for gate 66 and metallayer SD. The metal layer that is used in forming lower electrode 102 ofFIG. 5 may sometime be referred to as metal layer M2S. In addition tobeing used to firm lower electrode 102 of capacitor 100, metal layer M2Smay be used to form gate 110 of transistor 60.

In the configuration of FIG. 5, metal layer M2S has been formed ondielectric layers 68 and 70. Dielectric layer 132 is interposed betweengate 110 and semiconducting oxide channel 121 and serves as the gateinsulator for transistor 60. A passivation layer such as dielectriclayer 130 may be formed over channel 121 to protect the semiconductingoxide interface of channel 121. Dielectric layer 130 and dielectriclayer 132 may each be formed from silicon oxide, silicon nitride,aluminum oxide, hafnium oxide, a single layer, multiple sublayers, orother insulating materials.

FIG. 6 shows another illustrative configuration for transistor circuitry74. In the arrangement of FIG. 6, circuitry 74 has three metal layers.Metal layer GATE is used in forming lower electrode 102 for capacitor100 and is used in forming gate 66 for silicon transistor 58. Metallayer SD is used in forming source-drain terminals 74, 76, 114, and 116.An additional metal layer, sometimes referred to as metal layer G2, isinterposed between metal layer SD and metal layer GATE. Metal layer G2may be used in forming upper electrode 104 in capacitor 100 and may beused in forming gate 110 in oxide transistor 60. Oxide transistor 60 ofFIG. 6 is a bottom gate transistor. Dielectric layer 70 serves as thegate insulator for transistor 60 and is interposed between gate 110 andsemiconducting oxide channel 121. Passivation layer 130 may protectchannel region 121. In capacitor 100, dielectric layer 68 is interposedbetween upper electrode 104 and lower electrode 102.

In the illustrative configuration for circuitry 72 that is shown in FIG.7, upper electrode 104 of capacitor 100 is formed from metal layer SD.Metal layer SD may also be used in forming source-drain electrodes 74and 76 in silicon transistor 58 and source-drain electrodes 114 and 116in oxide transistor 60. Oxide transistor 60 may have a bottom gateconfiguration. Gate 110 of oxide transistor 60 and gate 66 of silicontransistor 58 may be formed from respective portions of the same metallayer (i.e., metal layer GATE). An additional metal layer (metal layerM2S) may be formed between metal layer GATE and metal layer SD. Metallayer M2S may be used in forming lower electrode 102 in capacitor 100.Dielectric layer 132 may be interposed between lower electrode 102 andupper electrode 104. Passivation layer 130 may be used to protect theinterface of semiconducting oxide layer 121 in oxide transistor 60.

It may be desired to minimize the inactive border region of display 14.Pixels 22 display images for a user, so the portion of display 14 thatis occupied by the array of pixels 22 forms the active area of display14. Portions of display 14 that surround the active area do not displayimages for a user and are therefore inactive. The amount of the inactivearea that is visible to a user can be minimize or eliminated by bendingportions of substrate 24 downwards out of the plane of the active area(e.g., at a right angle or at other suitable angles). To ensure thatdisplay 14 is not damaged during bending, the structures on substrate 24can be configured to enhance flexibility of display 14 in bent portionsof the inactive area. For example, insulating layers such as inorganicdielectric layers and other layers of display 14 (e.g., some of themetal layers) may be partly or completely removed in the inactive areato prevent stress-induced cracking or other damage during bending(particularly to metal signal lines).

Consider, as an example, display 14 of FIG. 8. As shown in FIG. 8,inactive edge area 204 has been bent downwards from active area 206about bend axis 200. Lines 202 (e.g., data lines or other metal signaltraces in display 14) traverse the bend at axis 200. To prevent theformation of cracks and other damage to the structures of display 14,some or all of the structures of display 14 other than lines 202 may beselectively removed in inactive area 204 (while being retained in activearea 206 to form thin-film transistor circuitry 72 such as circuitry 72of FIGS. 3, 4, 5, 6, and 7. With this approach, the metal layer thatforms lines 202 may be located at a greater distance above substrate 24in active area 206 than in inactive area 204.

To accommodate the disparity in height between the layers of active area206 and inactive area 204, a series of steps may be formed in thedielectric layers of display 14. The steps may slowly lower the heightof the metal traces that are supported on the dielectric layers, so thatthe metal traces can change height gradually and do not become cut offdue to a sharp height discontinuity in the dielectric.

An illustrative set of dielectric layers having a stepped profile sothat metal lines 202 can transition successfully between active area 206and inactive area 204 is shown in FIG. 9. As shown in FIG. 9, display 14may have dielectric layers such as layers L1, L2, and L3 (see, e.g., thedielectric layers of circuitry 72 in FIGS. 3, 4, 5, and 6). Layers L1,L2, and L3 may be formed from one or more sublayers of polymer and/orinorganic layers (e.g., silicon oxide, silicon nitride, hafnium oxide,aluminum oxide, etc.). There are three dielectric layers L1, L2, and L3in the example of FIG. 9, but this is merely illustrative. On the leftside of FIG. 9 in active area 206, all dielectric layers L1, L2, and L3are present, so metal line 202 is located at its maximum distance fromsubstrate 24. A staircase (stepped) dielectric profile is created byselectively removing layers L3, L2, and L1 at successively greaterlateral distances from active area 206. The steps in height that areformed in the dielectric layers allow metal line 202 to smoothlytransition from its maximum height (in active area 206) to its minimumheight in inactive area 204. Line 202 may, for example, rest on or nearthe surface of substrate 24 in inactive area 204.

FIG. 10 is a cross-sectional side view of illustrative thin-filmtransistor circuitry 72 for display 14 in a configuration in which upperlayers of material have been removed from the display to facilitatedisplay bending in a bend region along the inactive edge of the display.In the example of FIG. 10, all dielectric layers except passivationlayers 106 and 108 have been removed from substrate 24 in region 204, sometal lines 202 (e.g., data lines and/or other signal lines in display14) rest on the surface of substrate 24. This facilitates bending ofsubstrate 24 in region 204. In general, any suitable thin-filmtransistor circuitry 72 may be used with the inactive area materialremoval scheme of FIG. 10 (e.g., circuitry such as circuitry 72 of FIGS.3, 4, 5, 6, 7, and 8, etc.). The circuitry of FIG. 10 is merelyillustrative.

In the illustrative configuration of FIG. 10, upper capacitor electrode104 has been formed from metal layer M3. Metal layer M3 may also be usedin forming via 74 to couple source-drain terminal 74 to anode 44. Lowercapacitor electrode 102 may be formed from metal layer SD. Metal layerSD may also be used to form source-drain terminals 74, 76, 114, and 116.Passivation layers 106 and 108 (e.g., silicon nitride and silicon oxidelayers, respectively) or other suitable dielectric layer(s) may beformed on top of semiconducting oxide channel 112. In capacitor 100, oneof layers 106 and 108 may be locally removed to reduce dielectricthickness and thereby enhance the capacitance value of capacitor 100. Asshown in FIG. 10, for example, layer 106 may be removed under electrode104, so that layer 106 does not overlap capacitor 100 and so that onlydielectric layer 108 is interposed between upper electrode 104 and lowerelectrode 102 of capacitor 100. Dielectric layer 108 may be formed formsilicon nitride, which has a dielectric constant greater than that ofsilicon oxide, so the use of dielectric layer 108 as the exclusiveinsulating layer between electrodes 102 and 104 may help enhance thecapacitance of capacitor 100. An additional photolithographic mask maybe used to selectively remove silicon oxide layer 106. This mask mayalso be used in forming a dielectric step for metal lines 202 (see,e.g., the dielectric steps of FIG. 9). Metal lines 202 may be formedfrom metal layer SD. In active area 206 of display 14, metal lines 202may be formed from portions of metal layer SD that are supported bydielectric layers such as layers 122, 120, 64, 68, and 70 (i.e., layersof the type that may form illustrative layers L1. L2, and L3 of FIG. 9).Although there are three height steps in the example of FIG. 9, onestep, two steps, three steps, or more than three steps may be formed.

The illustrative configuration of FIG. 11 is similar to that of FIG. 10,but has an oxide transistor with a locally removed silicon nitridepassivation layer. Passivation layer 106 of FIG. 10 may be a siliconnitride layer. Silicon nitride layer 106 may have a high concentrationof hydrogen to passivate dangling bonds in polysilicon layer 62 ofsilicon transistor 58. For effective passivation, silicon nitride layer106 may overlap transistor 58 and silicon channel 62. It may bedesirable to prevent the hydrogen from silicon nitride layer 106 fromreaching semiconducting oxide channel 112. This can be accomplished byremoving nitride layer 106 from transistor 60. For example, aphotolithographic mask may be used to pattern silicon nitride layer 106so that silicon nitride layer 106 is absent under semiconducting oxide112 (i.e., so that there is no portion of nitride layer 106 thatoverlaps transistor 60). By ensuring that no silicon nitride is presentbetween gate 110 and oxide 112, the performance of transistor 60 willnot be degraded due to hydrogen from layer 106.

The foregoing is merely illustrative and various modifications can bemade by those skilled in the art without departing from the scope andspirit of the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. An organic light-emitting diode display,comprising: a substrate; an array of pixel circuits that form an activearea of the substrate; circuitry in an inactive area of the substrate;dielectric layers that have a first thickness in the active area, arenot present in the inactive area, and have a second thickness that isless than the first thickness between the active area and the inactivearea, wherein each pixel circuit comprises: an organic light-emittingdiode; a silicon transistor coupled in series with the organiclight-emitting diode; a storage capacitor coupled to the silicontransistor; and a semiconducting oxide transistor coupled to the storagecapacitor.
 2. The organic light-emitting diode display defined in claim1 wherein the substrate is bent in the inactive area.
 3. The organiclight-emitting diode display defined in claim 2 wherein the silicontransistor in each pixel circuit comprises a silicon channel, whereinthe dielectric layers include a buffer layer between the substrate andthe silicon channel, and wherein the buffer layer is not present in theinactive area.
 4. The organic light-emitting diode display defined inclaim 3 further comprising a first metal layer in the active area,wherein some of the first metal layer forms a gate for the silicontransistor in each pixel circuit.
 5. The organic light-emitting diodedisplay defined in claim 4 wherein some of the first metal layer forms agate for the semiconducting oxide transistor in each pixel circuit. 6.The organic light-emitting diode display defined in claim 4 furthercomprising a second metal layer, wherein the second metal layer ispatterned in the active area to form source-drain terminals for thesilicon transistor and for the semiconducting oxide transistor.
 7. Theorganic light-emitting diode display defined in claim 6 wherein thesecond metal layer is patterned in the inactive area to form data linesthat are coupled between the array of pixel circuits and the circuitryin the inactive area.
 8. The organic light-emitting diode displaydefined in claim 7 wherein the substrate is a bent flexible substrateand wherein the data lines are bent and are formed on a surface of thesubstrate so that none of the dielectric layers are interposed betweenthe data lines and the substrate.
 9. The organic light-emitting diodedisplay defined in claim 8 wherein the semiconducting oxide transistorin each pixel comprises a semiconducting oxide channel.
 10. The organiclight-emitting diode display defined in claim 9 wherein the dielectriclayers include a silicon nitride layer that overlaps the silicon channelof the silicon transistor in each pixel circuit and that does notoverlap the semiconducting oxide channel of the semiconducting oxidetransistor in each pixel circuit.
 11. The organic light-emitting diodedisplay defined in claim 10, wherein the storage capacitor has a firstelectrode formed from the second layer of metal and has a secondelectrode.
 12. The organic light-emitting diode display defined in claim11 wherein the dielectric layers include an additional silicon nitridelayer, wherein the additional silicon nitride layer is interposedbetween the first and second electrodes of the storage capacitor in eachpixel circuit.
 13. The organic light-emitting diode display defined inclaim 12 further comprising a silicon oxide layer that overlaps thesemiconducting oxide channel in each pixel circuit and that is locallyremoved in the storage capacitor of each pixel circuit so that none ofthe silicon oxide layer is interposed between the first and secondelectrodes of the storage capacitor.
 14. The organic light-emittingdiode display defined in claim 2 further comprising data lines thatextend from the active area to the inactive area, wherein the dielectriclayers have a stepped profile that reduces from the first height to thesecond height when transitioning from the active area to the inactivearea, and wherein the data lines are formed on the dielectric layerswith the stepped profile.
 15. The organic light-emitting diode displaydefined in claim 1 wherein the semiconducting oxide transistor in eachpixel circuit comprises a drive transistor and wherein the silicontransistor in each pixel circuit comprises a switching transistor. 16.An organic light-emitting diode display, comprising: a flexible polymersubstrate; an array of pixel circuits on the substrate, each pixelcircuit including an organic light-emitting diode, at least twosemiconducting oxide transistors each having a semiconducting oxidechannel, at least one silicon transistor coupled in series with theorganic light-emitting diode, and at least one storage capacitor;dielectric layers on the flexible polymer substrate that have a steppedprofile that reduces in height when transitioning from the array ofpixel circuits to an inactive area adjacent to the array of pixelcircuits; and data lines on the dielectric layers that follow thestepped profile, wherein the dielectric layers include a dielectriclayer that overlaps the at least one silicon transistor and that doesnot overlap the semiconducting oxide channels.